It is known to form a layer of a hydrophilic polyurethane resin which is substantially non-porous, on a porous base material such as cloth, to produce a moisture permeable material suitable for application to e.g. a clothing material. The layer of this hydrophilic polyurethane resin absorbs moisture from a high humidity side and discharges moisture to a low humidity side thereby to provide a moisture permeability.
In order to form such a layer of a hydrophilic polyurethane resin, it is common to employ a method wherein a solution or dispersion of the hydrophilic polyurethane resin is applied on a porous base material to form a coating layer, a method wherein a coating agent selected from a mixture of raw materials for the hydrophilic polyurethane resin and a solution and dispersion thereof, is directly applied on a porous base material to form a coating layer, or a method wherein the coating agent is applied on a base material having a release property to form a non-cured coating layer which is not completely cured, and then such a non-cured coating layer is laminated on a porous base material, followed by curing.
As an intermediate of the hydrophilic polyurethane resin in the coating agent, it has been proposed to employ an isocyanate group-containing prepolymer obtainable by reacting a polyol mixture containing a polyoxyethylene polyol with a polyisocyanate compound (e.g. the following Patent Documents 1 and 2).
In Example of Patent Document 1, the polyol mixture used in preparation of an isocyanate group-containing prepolymer is a mixture having 80 parts of polyoxyalkylene triol with a hydroxy value of 47.0 and 700 parts of polyoxyalkylene diol with a hydroxy value of 56.1. In this polyol mixture, the average number of hydroxy groups is about 2.1, and the molecular weight per hydroxy group is about 1,000.
The method described in Patent Document 2 is characterized by using a polyoxyalkylene type polyol mixture which satisfies 2.7<n<3.6 where n is the average number of hydroxyl groups, and has an average oxyethylene group content of from 60 to 90 wt %, in the synthesis of the isocyanate group-containing prepolymer. The polyoxyalkylene type polyol mixture used in Example 4 (Comparative Example) in Patent Document 2, has an average number of hydroxy groups of 2.6, and a molecular weight per hydroxy group of about 1,093.